Homebrew

Traits

Magic Weapons. The Behemoth's weapon attacks are magical.   Regeneration. The Behemoth regains 15 hit points at the start of its turn if it has at least 1 hit point. If it takes Piercing damage it only regenerates 7 hit points.

Actions

Multiattack. The Behemoth makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: 1d20+10 to hit, reach 5 ft., one target. 1d8+6 piercing damage plus 3d6 psychic damage.
Claws. Melee Weapon Attack: 1d20+10 to hit, reach 5 ft., one target. 1d10+6 slashing damage plus 2d6 psychic damage.

Psionics. It uses its psychic abilities as spell like effects. It has a Spell save DC of 16.

Antagonize. It fills a target with rage and causes it to lash out to a target nearby. The creature makes a Wisdom Save DC16, on failure it takes 6d6 psychic damage and must make a melee attack against a creature in range, on success they take half damage and resist making an attack.

Dominate Monster. (Psionic) You invade a creatures mind putting its consciousness in a nightmare of something they fear the most. The creature makes a Wisdom Saving Throw DC16, on failure the creature is Dominated by the Behemoth and can send telepathic commands to it as a free action. It can use its Reaction to force the creature to act outside of its turn to use a Reaction. If the creature is damaged they reroll the Wisdom Saving Throw.

Mind Assault. The targeted creature's mind is filled with whispers and voices causing a major headache. The creature makes a Wisdom Save DC16, on failure it takes 4d6 psychic damage and has Disadvantage on its next attack. On success, it takes half damage.

Mental Feedback. Recharges on a 5 or a 6. All creatures within a 20 ft radius of the Behemoth must make a Wisdom Saving Throw DC16, on failure they take 4d6 psychic damage and their movement speed become 0 until the end of their next turn. On success they take half damage and only lose 10 feet of movement.

Legendary Actions

It has one Legendary Action per round.

Antagonize. It fills a target with rage and causes it to lash out to a target nearby. The creature makes a Wisdom Save DC16, on failure it takes 6d6 psychic damage and must make a melee attack against a creature in range, on success they take half damage and resist making an attack.